Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
A review of assessments conducted on bio-ethanol as a
transportation fuel from a net energy, greenhouse gas,
and environmental life-cycle perspective
Harro von Blottnitz*
Chemical Engineering Department, University of Cape Town, Private Bag, 7701 Rondebosch, South Africa
e-mail: hvb@chemeng.uct.ac.za
and
Mary Ann Curran
Environmental Protection Agency, National Risk Management Research Laboratory, Cincinnati, Ohio 45268 USA
e-mail: curran.maryann@epa.gov
* Corresponding author
Abstract
Interest in producing ethanol from biomass in an attempt to make transportation
ecologically sustainable continues to grow. In recent years, a large number of assessments
have been conducted to assess the environmental merit of biofuels. Two detailed reviews
present contrasting results: one is generally unfavourable, whilst the other is more favourable
toward fuel bio-ethanol. However, most work that has been done thus far, to assess the
conversion of specific feedstocks to biofuels, specifically bio-ethanol, has not gone beyond
energy and carbon assessments. This study draws on 47 published assessments that compare
bio-ethanol systems to conventional fuel on a life-cycle basis, or using life cycle assessment
(LCA). A majority of these assessments focused on net energy and greenhouse gases, and
despite differing assumptions and system boundaries, the following general lessons emerge:
i) make ethanol from sugar crops, in tropical countries, but approach expansion of
agricultural land usage with extreme caution; ii) consider hydrolysing and fermenting
lignocellulosic residues to ethanol; and iii) the LCA results on grasses as feedstock are
insufficient to draw conclusions. It appears that technology choices in process residue
handling and in fuel combustion are key, whilst site-specific environmental management
tools should best handle biodiversity issues. Seven of the reviewed studies evaluated a wider
range of environmental impacts, including resource depletion, global warming, ozone
depletion, acidification, eutrophication, human and ecological health, smog formation, etc.,
but came up with divergent conclusions, possibly due to different approaches in scoping.
These LCAs typically report that bio-ethanol results in reductions in resource use and global
warming; however, impacts on acidification, human toxicity and ecological toxicity,
occurring mainly during the growing and processing of biomass, were more often
unfavourable than favourable. It is in this area that further work is needed.
Keywords: bio-ethanol, life cycle assessment, energy balance, greenhouse gas, sustainable
transportation
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
1. Introduction
Ethanol derived from biomass is often advocated as a significant contributor to possible
solutions to our need for a sustainable transportation fuel. Kim and Dale [1] estimated that
the potential for ethanol production is equivalent to about 32% of the total gasoline
consumption worldwide, when used in E85 (85% ethanol in gasoline) for a midsize
passenger vehicle. Such a substitution immediately addresses the issue of reducing our use of
non-renewable resources (fossil fuels) and the attendant impacts on climate change,
especially carbon dioxide and the resulting greenhouse effect, but it does not always address
the notion of overall improvement. For instance, it is well understood that the conversion of
biomass to bio-energy requires additional energy inputs, most often provided in some form
of fossil fuel. The life cycle energy balance of a biofuel compared to conventional fossil fuel
should be positive, but depending on the processing choices, the cumulative fossil energy
demand might, at times, only be marginally lower or even higher than that of liquid fossil
fuels (e.g., [2, 3]). Also, ethanol in gasoline may result in decreased urban air quality, and be
associated with substantive risks to water resources and biodiversity [4].
Bio-based systems have other possible ecological drawbacks. Agricultural production of
biomass is relatively land intensive, and there is a risk of pollutants entering water sources
from fertilisers and pesticides that are applied to the land to enhance plant growth. A very
large number of researchers have recognised this conundrum and have attempted to analyse
bio-ethanol systems in an effort to describe their environmental sustainability and to
determine whether bio-based fuels, i.e. biofuels, are helping us achieve the goal of providing
environmentally sustainable transportation. Two recent reviews have attempted to
summarise the findings. One focused on ethanol alone and presents generally unfavourable
recommendations [4]; the other review looked at biofuels more generally and presented
more favourable results for ethanol but cautioned with respect to some of its environmental
impacts [5]. It must be noted that a number of studies that looked specifically at the North
American corn-to-ethanol route, were very critical as to its environmental sustainability [3, 6,
7].
Whilst the issue of sustainability is complicated, one that encompasses human and
environmental health as well as societal needs, it is clear that our efforts to identify solutions
should be broad in scope to avoid shifting problems from one place to another [8]. A large
number of authors have studied liquid biofuel production systems, both current and
projected, with the aim of determining whether the currently accepted premise that such
systems contribute to environmental sustainability is valid. In this paper we review previous
evaluations of bio-ethanol (as a transportation fuel) that used life cycle thinking or life cycle
assessment as the basis for the evaluation. It is assumed that the reader has a fundamental
knowledge of bio-ethanol production systems, so such background information is not
provided here. The paper begins with a brief review of the study approach, then provides an
overview of the evaluations that were found in a search of the open literature, and concludes
with a summary of key findings and recommendations both for policy on bio-ethanol
projects, and for further studies.
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
2. Approach used in this study
2.1 Objective
The objective of the study was to review recent evaluations of bio-ethanol, made from
varying feedstocks for use as a transportation fuel, compared to conventional fuels on a life
cycle basis. The effort consisted of a literature search and a desk study, followed by an
analysis of the methods and assumptions used, and findings obtained to detect if any trends
could be identified in the results when viewed by the type or location of the feedstock.
2.2 Scope of the search
An online search of publicly-available papers and reports was conducted to find studies that
have been published in recent years (1996 – 2004). The focus of the search was on ethanol
from biomass for use as a transportation fuel (a gasoline replacement). The search included
completed, published assessments that claimed to be life-cycle based and that were
environmental in nature. Cost analyses were not part of the main focus of the study. Only
those reports that are available in English were subjected to further analysis; 47 reports were
included in the analysis.
This area of research is still of significant interest worldwide and studies on biofuels continue
to be conducted. Although additional studies have been published since the completion of
the literature search, this paper includes the assessments that were available at that time.
2.3 Defining the life cycle
Life cycle management is quickly becoming a well-known and often used approach for
environmental management. A comprehensive environmental assessment of an industrial
system needs to consider both upstream and downstream inputs and outputs involved in the
delivery of a unit of functionality. A life cycle approach involves a cradle-to-grave
assessment, where the product is followed from its primal production stage involving its raw
materials, through to its end use. The diagram in Figure 1 illustrates a generic biofuel lifecycle scheme; it shows the main sub-processes, and identifies the flows of importance for
describing environmental performance.
INSERT FIGURE 1
The main stages A to E can be studied in order to determine the holistic performance of the
system, depending on the goals of the study. It is at this point that differences in studies that
are called life cycle assessments can be seen. Some studies include cradle-to-grave boundaries
but evaluate limited input or output data. Most often, studies on energy and carbon balances,
as well as greenhouse gas emissions, are found in the literature. The goal of a life cycle
assessment (LCA), on the other hand, is to model all potential impacts to human health and
the environment across all media - air, water and solid waste (see Appendix at the end of the
paper for a longer discussion on LCA). A distinction can then be made between studies that
are life-cycle based versus those that aim to be fuller life cycle assessments.
3. Overview of published studies
The on-line literature search led to a recent review study that was conducted by the Institute
for Energy and Environmental Research (IFEU) with a similar objective [5]. This study
analyzed and compared all international, publicly-accessible publications about biofuels that
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
are currently used for transportation (e.g., bio-diesel and bio-ethanol as well as those
potential fuels like biomass-to-liquid, BTL). The literature search uncovered additional
references that were not part of the IFEU review. The integration of these efforts resulted in
47 publications, in English, that address bio-ethanol (see Table 1). Note that whilst several
studies encompassed the entire life cycle as depicted in Figure 1, many studies did not extend
beyond ethanol production. It was nevertheless possible to compare studies with such
differing system boundaries, at least for the carbon and energy analyses, by developing a
spreadsheet to reflect all the E and C streams in Figure 1. For those studies that exclude life
cycle stages such as fuel distribution, storage and combustion (in use), it was then assumed
that the carbon and energy flows associated with these stages were similar to results
documented in other studies.
To date, the emphasis in life cycle based studies of bio-ethanol has strongly, but not
exclusively, focused on North America and Europe, and the few full LCAs completed
also do not cover the full range of possible or promising options.
INSERT TABLE 1
4. Key results from selected bio-ethanol system assessments
Results are discussed in three categories of special interest to the question of environmental
sustainability: (1) reducing dependence on fossil fuels through energy balance assessments;
(2) reducing emissions of greenhouse gases (GHGs), and (3) reducing health and
environmental impacts throughout the life cycle. Each interest area is discussed in more
detail in the following sections.
4.1 Energy balance assessments
Almost all studies on biofuels consider the question as to whether such fuels achieve the
desired net effect of lowering the amount of fossil fuel needed to propel standard and nearfuture vehicles powered by spark-ignition internal combustion engines. As discussed in the
appended methodological discussion on energy assessments, a variety of indicators has been
developed for this purpose, and it is important to norm these to the few most appropriate
ones. The IFEU review [5] does this well.
Whilst this type of analysis is often inspired by the controversial results of Pimentel on
ethanol from corn in the United States (e.g., [9]), the bulk of the studies report moderate to
strong fossil fuel substitution effects for bio-ethanol systems. This is evident from Figure 2
and Table 2, that present results for two of the most commonly used energy balance
indicators.
4.1.1 Net replaced fossil energy
This indicator can be reported relative to the achieved transportation effect (e.g., per
kilometre driven) or relative to the land area used, as is done in Figure 2. It must be noted
that no additional land is needed when by-products (e.g., molasses) or lignocellulosic residue
are used as feedstock for fermentation. In this regard, for these latter feedstocks, Figure 2
indicates the potential amounts of replaced fossil energy per hectare of land, but this is not
an additional land requirement as it is for the food crops in the lowest section of the figure.
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
Of the possible sources of bio-ethanol, sugar crops are most land-efficient in replacing fossil
energy, and here tropical sugarcane significantly outperforms sugar beet in temperate
regions. Our interpretation of the Brazilian studies (at about 250 GJ/hectare-annum) appears
somewhat less conservative than that of Quirin et al. [5], which may be due to the inclusion
of by-product electricity credits.
Starch crops, such as maize (corn), potatoes, wheat and rye, replace significantly less fossil
energy. The IFEU study [5] reports a range of 35-50 GJ/ha.a; studies which we are citing
compare well with this range, at 35-40 GJ/ha.a for potato and wheat [10], and a projected
27-56 GJ/ha.a for wheat and rye winter crops [11]. An unusually high result is reported by
Hanegraaf et al [12], who reported 124 GJ/ha.a for winter wheat, but yielding heat and
power in addition to ethanol. For corn in North America, our analysis of the definitive
USDA study [13] yields a fossil energy replacement of 38 GJ/ha.a – a number of much
debate, although its poor performance relative to the sugar crops is not doubted by any
commentators.
INSERT FIGURE 2
For ethanol made from a waste product taken to carry no environmental burden, a fossil
energy replacement can also be determined on a per hectare basis. However, in interpreting
these, it must be remembered that this is not the additional land area needed, but rather an
additional bio-energy contribution that can potentially be harnessed from land already in use.
Results will differ on a case-by-case basis, depending on how efficiently wastes and byproducts are already used, and how the industrial systems are configured. The two diverging
results for molasses illustrate this: the Indian case yielding 30 GJ/ha.a is for a distillery fully
integrated into a sugar mill, where excess low pressure steam is used; whereas the South
African case yielding 5 GJ/ha.a is for a distillery distant from sugar mills, relying on coal and
grid electricity for its energy needs.
For ethanol from lignocellulosic feedstocks, the contribution to fossil energy replacement is
of a similar magnitude to that of the starch crops. Our interpretation of studies on sugarcane
bagasse, corn stover and wheat straw here agrees well with the range reported by Quirin et
al. [5], at 25-90 GJ/ha.a. It is important to note that the three studies we refer to are all for
waste lignocellulosic material. Dedicated energy cropping (e.g., of grasses) is a future
possibility that needs to be considered too.
4.1.2 Energy yield ratios
The ratios relating energy output of the resultant biofuel to the fossil energy input into its
production are also often used to test the sensibility of making a particular product. Table 2
summarises our analysis of key studies for a range of feedstocks and locations in this regard.
Again, the tropical sugarcane-based ethanol production outperforms that from starch crops
in temperate regions by a significant margin. Several commentators have questioned whether
the energy yield ratio for ethanol from corn in the US is at all positive, though the balance of
evidence seems to indicate it is, if only marginally [13].
INSERT TABLE 2
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
In the case of molasses utilization, the two studies which we cite yield very diverging ratios –
the physical differences between the two cases have already been discussed above; in
addition, it is probable that the Indian study has neglected several non-factory inputs of
fossil energy into the system.
In the case of utilization of lignocellulosic wastes through hydrolysis and fermentation, the
cited studies all project very encouraging bio-energy yields in relation to the required fossil
energy inputs.
Concerning the effectiveness of bio-ethanol to replace fossil energy, Quirin et al. [5] have
concluded that the desired effect is generally achieved, and our more limited review confirms
this. It is, however, also clear that tropical sugarcane-based production is most effective from
this vantage point.
4.2 Greenhouse gas assessments
With scientific evidence now increasingly mounting that climate is changing, and that this
can be attributed to the large-scale use of fossil fuels, the potential of biofuels to deliver
transportation energy in a carbon-neutral way is receiving increasing attention. Most studies
on bio-ethanol systems have thus, also investigated at least their CO2 balance, and often also
those of the other major greenhouse gases methane and nitrous oxide. Again, a multitude of
different indicators are used, and results are often not immediately comparable.
4.2.1 Avoided CO2 equivalent emissions from bio-energy systems
Closely related to the replaced fossil energy indicator is the avoided emission of greenhouse
gases (GHGs). It is dominated by CO2 flows, but the nature of the replaced fossil fuels (coal,
oil, gas) does introduce a degree of divergence from the energy indicator, as these fuels are
characterised by different fossil carbon intensities. Careful accounting for the two next
important GHGs, i.e. CH4 and N2O, may exacerbate this variation, with global warming
potentials of 21 and 310 times those of CO2 respectively.
Again, the avoided CO2 indicator can be derived relative to a kilometre driven, or to the land
area used. Figure 3 presents the results of our limited evaluation of avoided GHG emissions
per hectare cropped and year, for the same studies as in Figure 2, and compares our results
with those by Quirin et al. [5].
INSERT FIGURE 3
Sugar-based production systems again achieve much higher effects per hectare of cropped
land than starch-based systems, and tropical sugarcane is again by far the most efficient crop.
Our analysis yields a much higher figure for avoided GHGs than that of Quirin et al. [5],
again because of our inclusion of the substitution effect of bio-based process heat and
electricity. For the other feedstocks, our interpretation of the selected studies agrees well
with the more general results of Quirin et al [5].
4.3 Health and environmental impact assessments
Only seven of the reviewed studies listed in Table 1 evaluate impacts that are more
expansive in scope than the studies described in the previous sections. Whilst these studies
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
all account for energy (as resource demand), CO2 and greenhouse gas emissions, they go
beyond these measures and include additional impact indicators. As each of these studies
had a somewhat different objective and therefore, also a different scope, we have not
attempted to harmonise the results as in the previous sections, but have opted to rather
individually summarise each of them in the following paragraphs and in Table 3. Full
citations are included in the references section.
INSERT TABLE 3
Table 3 summarises the findings of these seven LCA studies by indicating for 13 impact
categories and 6 related inventory categories whether the study reports an increased or
decreased impact for bio-ethanol compared to conventional fuel. A dash indicates no
change. In cases where only inventory data were provided, the relevant impact category was
applied and interpreted as an increase, decrease or no change. As one scans across the lines
of this table, it becomes evident that there is not much consensus on the environmental
benefits of fuel bio-ethanol beyond the broad agreement that they do avoid to some extent
the use of fossil energy carriers, and consequently also reduce GHG emissions.
Kadam (2002). Environmental benefits on a life cycle basis of using bagasse-derived
ethanol as a gasoline oxygenate in India [14].
Feedstock: Bagasse
Location: India
Basis:
1 dry tonne of bagasse to produce 10% by volume ethanol in gasoline (E10).
System description:
The study compares the conventional practice of burning bagasse in the field and using
conventional fuel (Scenario 1) to a hypothetical process of converting bagasse into ethanol
for use in E10 (Scenario 2). Boundaries include bagasse transport, ethanol production, use
and excess electricity.
Impacts:
- Non-renewable resource depletion
- Greenhouse effect
- Air acidification
- Eutrophication
- Human toxicity
- Waste generation
- Air odour
Findings:
The author concludes that there are significant benefits in diverting excess bagasse to
ethanol production as opposed to the current practice of open-field burning. Scenario 2
leads to a decrease in carbon monoxide, hydrocarbons, SOx, NOx, particulates, carbon
dioxide, methane and fossil fuel consumption. COD (from ethanol raw material production)
is significantly higher. Non-methane hydrocarbons are from ethanol production. Lime,
ammonia & sulphuric acid occur only in Scenario 2. Electricity credits result in negative CO2
and CH4 emissions and lower solid waste.
Kaltschmitt, Reinhardt & Steltzer (1997). Life cycle analysis of biofuels under
different environmental aspects [10].
Feedstock: Sugar beet, wheat, and potato
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
Location: Germany
Basis: 1 hectare
System description:
This study compared bio-based systems, including cultivation and harvesting of raw
materials, through energy use, to fossil systems, including mining and processing of raw
materials through energy use.
Impacts:
- Finite energy
- Global warming potential (CO2 equivalents)
- Nitrous oxide
- Acidification potential
- Sulphur dioxide
- Nitrogen oxide
Findings:
The study shows some clear ecological advantages of bio-ethanol over fossil fuels, such as
conserving fossil energy sources and reducing global warming potential, but bio-ethanol also
has some definite disadvantages; in particular N2O and NOX emissions are higher. SO2
emissions and, correspondingly, acidification potential show no discernible change.
Puppan (2002). Environmental evaluation of biofuels [15].
Feedstock: Sugar beet, winter wheat, and potato
Location: Germany
Basis:
Summary of a German study on E5 fuel versus gasoline [16].
System description: Not provided
Impacts:
- Depletion of abiotic resources
- Climate change
- Stratospheric ozone depletion
- Acidification
- Human & Ecotoxicity
Findings:
For the bio-ethanol portion of the paper, Puppan cites a German study [16] that shows that
E5 (5% ethanol) fuel has lower impacts for depletion of abiotic resources and climate
change, but higher impacts for stratospheric ozone depletion (acidification and human
toxicity impacts were mostly unchanged). Puppan states that the LCA study proved the
environmental benefit of biofuels during the combustion in the engine, but also emphasised
the environmental drawbacks that occur during the agricultural phase, such as pollution of
ground and groundwater by fertilisers and pesticides as well as the creation of monocultures.
Puppan concludes that it is apparent that the net environmental impact significantly depends
on the agricultural conditions.
Reinhardt & Uihlein (2002). Bio-ethanol and ETBE (ethyl tertiary butyl ether) versus
other biofuels for transportation in Europe: an ecological comparison [17].
Feedstock: Sugar beet, wheat and potato
Location: Europe
Basis:
Per kilometre
System description:
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
The study includes fertiliser, fuel, and pesticide production; cultivation; sugar extraction;
ethanol production; and consumption (use in the vehicle).
Impacts:
- Resource demand (natural gas, mineral coal, brown coal, uranium ore)
- Greenhouse gas emissions (CO2, CH4, N2O)
- Acidification
- Eutrophication
- Photochemical smog (N2O)
- Human toxicity (reported as LCI)
- Eco toxicity (reported as LCI)
Findings:
For all life cycle comparisons, resource demand and greenhouse gas effect are in favour of
biofuels, whereas most of the other parameters are in favour of the fossil fuels. Ethanol
from sugar beets has advantages over wheat and potato.
Hu et al (2004). Economics, environment, and energy life cycle assessment of
automobiles fueled by bio-ethanol blends in China [18].
Feedstock: Cassava
Location: China
Basis:
200,000 kilometre driving distance
System description:
Cassava, from the Guangxi Province, is converted to E85 fuel for use in a 5-passenger
vehicle.
Impacts:
The environmental impacts are reported as inventory releases of CO2, CO, hydrocarbons
(HC), NOx, and particulate matter (PM).
Findings:
The cassava-based E85 fuel has lower life cycle CO2, CO, HC, and PM pollutants than
gasoline fuel; however, it has higher NOx emissions. The combined environment indicator
is calculated to be 20% lower for bio-ethanol.
Sheehan et al (2004). Energy and environmental aspects of using corn stover for fuel
ethanol [19].
Feedstock: Corn stover
Location: USA (Iowa)
Basis: 1 hectare of land and 1 kilometre travelled using 85% ethanol in gasoline (E85) versus
gasoline.
System description:
Sheehan describes a hypothetical system of using corn stover to make E85. The processes
include stover production & collection; transport; ethanol production; distribution; and use.
The system also includes the gasoline system, with which the ethanol is blended, from crude
oil extraction through use.
Impacts:
- Fossil energy use
- Greenhouse gas emissions
- Air quality (ozone precursors; CO; NOx)
- Land use (soil health)
- Cost
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Findings:
Findings are presented in the paper for a few key metrics:
- Fossil energy use is 102% and greenhouse gas emissions are 113% lower for E85.
- 2.91 MJ/km avoided non-renewable energy.
- Air quality impact is mixed with emissions of CO, NOX, and SOX substantially higher. NOX
emissions result mainly from farm soil. SOX emissions result from the combustion of lignin
residue at ethanol plants. Hydrocarbon ozone precursors are reduced.
- Stover can be removed from the field whilst maintaining or increasing soil carbon.
Tan & Culuba (2002). Life-cycle assessment of conventional and alternatives fuels for
road vehicles [20].
Feedstock: Cellulosic agricultural waste using enzymatic hydrolysis and fermentation
Location: Philippines
Basis:
Per kilometre
System description:
The LCA encompasses extraction of raw materials and energy resources; conversion of these
resources into the desired product; the utilization of the product by the consumer; and the
disposal, reuse, or recycling of the product after its service life.
Impacts:
- Resource depletion (oil, coal, natural gas)
- Human toxicity potential (PM10)
- Nutrification
- Photochemical ozone
- Acidification
- GWP (CO2, CH4, N2O)
- Air emissions (VOC, CO, NOX, PM10, SOX)
Findings:
For Scenario A, using Philippine Department of Energy projections for the year 2009, the
use of bio-ethanol in place of gasoline is expected to yield significant gains particularly with
respect to fossil fuel depletion and greenhouse gas emissions. The total impacts for bioethanol are significantly lower than those of gasoline, primarily due to sharp reductions in
CO2 emissions (and global warming potential) and fossil fuel consumption. Tan & Culuba
state that impacts of biofuels in other impact categories remain roughly comparable to those
of conventional fuels (Table 1 shows acidification, nitrification and human toxicity potentials
that are slightly larger and photochemical oxidation potential slightly less than conventional
fuel).
5. Findings and Recommendations
Published life cycle based assessments of the sustainability of bio-ethanol systems have
investigated a wide variety of feedstocks (as presented in Table 1). An array of different
metrics has been used to convey their results, sometimes complicating comparisons.
Methods have varied from simple energy and carbon accounting to attempts to be more
inclusive in addressing sustainability. Much of the focus has been to determine if the use of
biomass to make fuel is a net loss or a net gain regarding energy input versus output.
Two factors emerge as dominating the energy performance of bio-ethanol systems:
crop/climate productivity, and nature of the feedstock. With regard to both of these, it is
highly significant that both tropical sugar crops (by far the most productive) and cellulosic
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
feedstocks (potentially most sustainable and abundant), have, to date, received the least
amount of attention in bio-ethanol sustainability assessments that go beyond energy and
carbon analysis.
The overriding conclusion of the studies that looked at energy balances was that the use of
bio-ethanol in place of conventional fuels or as an additive leads to a net gain. That is, the
prevailing data indicate that it takes less energy to make and distribute ethanol than can be
delivered by the fuel. The results of the studies that evaluated other environmental impact
categories beyond energy and greenhouse gases were mixed. Acidification, human toxicity
and ecological toxicity impacts, mainly occurring during the harvesting and processing of the
biomass, were more often unfavourable than favourable for bio-ethanol. The IFEU study
had similar findings and concluded that for all life cycle comparisons, resource demand and
GHG effect are in favour of biofuels, whereas most of the other parameters they evaluated
are in favour of fossil fuels [17].
Our recommendations for future sustainability assessments of bio-ethanol are:
1. It is not necessary to repeat detailed energy and GHG assessments. Depending on
crop and geographical location, in many cases it will be possible to obtain a
sufficiently reliable estimate from previous work (e.g., [5] or from the Biomitre
website [21]).
2. Studies should be selected to fill the critical gaps: full life cycle assessments are
needed on ethanol from tropical sugar crops, and on 2nd generation bio-ethanol from
cellulosic cropped feedstocks, such as perennial grasses or short rotation forests.
3. The assessments must be cradle-to-grave, as significant air quality impacts may be
associated with the bio-ethanol used in internal combustion engines.
4. Attention must be paid to gathering the data needed for the disputed environmental
categories of acidification, eutrophication, photochemical smog, human and ecotoxicity, as well as land use and its effects on biodiversity. Put another way, the
safeguard subjects of human and ecological health need to feature more prominently
next to those of climate change and resource depletion concerns.
5. Data gaps for life cycle assessments of corn to bio-ethanol in the United States
should be addressed and filled, to address shortcomings of studies, to date, in
accordance with recommendations 3 and 4.
6. Conclusion
Moving toward sustainability requires a re-thinking of our systems of production,
consumption and waste management and an increased awareness of the need to avoid
shifting of problems, as often occurs with isolated measures. The ecological advantages
should outnumber, or outweigh, the disadvantages to the environment and human health.
Numerous studies have been done in recent years evaluating the life cycle impacts of bioethanol, and there is now strong evidence that all bio-ethanol production is mildly to
strongly beneficial from a climate protection and a fossil-fuel conservation perspective. Fuel
ethanol produced from sugar crops in tropical settings appears by far the most efficient in
these categories from a land-use perspective. However, whilst over 40 studies have been life
cycle based, only seven were identified which could be said to approach life cycle
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
assessments. These studies do not, of course, cover the full range of possible feedstocks and
geographies, and their results in the standard impact categories diverge. Further assessments
should thus, take energy and carbon performances as understood, work on the less studied
but highly promising feedstocks and locations outside Europe and North America, and pay
more attention to the safeguard subjects of human and ecological health. We caution against
basing fuel production policy on environmental sustainability studies that are life cycle based
in the sense of extending from the crop to the wheel, but that ignore issues other than fossil
fuel depletion and GHG emissions; such practices are likely to result in detrimental shifting
of burdens.
Disclaimer
This research was supported in part by the U.S. Environmental Protection Agency
through its Office of Research and Development. This paper has not been subjected to
Agency review and therefore does not necessarily reflect the views of the Agency, and no
official endorsement should be inferred.
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Appendix:
Energy analysis approaches: input versus output
The energy analysis approach evaluates all the fossil fuel inputs in upstream processing steps
like agriculture, transportation and processing, and these are compared against the delivered
energy of the product biofuel. Referring to the terminology in Figure 1, the net energy
available from a fuel, Ee, is equal to (EG-Ee1), where EG is the gross energy produced by the
fuel during combustion and Ee1 is the total feedback energy in the fuel production process.
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
Proposed in the literature are energy yield ratios, e.g., the ratio of gross energy output to
energy input (Eg/Ei, when there is no fossil energy input as in the case described by Prakash
et al [22] or Eg/(Ei+Eb+Ec+Ed) in the more general case).
Similarly, a fossil energy ratio is proposed by Sheehan et al [23], defined as
(Enet + Ecop)/ (EA + EB + EC + ED)
To avoid any confusion, we will here call this the bio-energy yield to fossil energy input ratio
(or ByFi ratio). This relates the energy retrieved from a product biofuel, weighed against the
fossil energy input involved in its life cycle, particularly in its production and conversion, and
the related upstream processes. It is observed that for fossil energy ratios greater than 1, the
system approaches renewability, which is theoretically only feasible for no fossil energy
requirements (ratio of infinity). It might be more useful to describe this ratio as a
“bioenergy ratio,” as its value increases as the fossil energy input to the system decreases.
The use of fossil energy replaced should also be of interest – especially when comparing
liquid fuel options to other bioenergy scenarios, such as electricity generation, and it is
reported more frequently in recent studies. This measure is the total energy needed to
provide an equivalent of amount of gasoline less all fossil energy uses needed to produce the
bio-ethanol:
Avoided fossil energy = EE + Ecop – (EA+EB+EC+ED)
Carbon balancing approaches
Carbon dioxide is the key greenhouse gas responsible for environmental issues of climate
change. The production and use of agro-based fuels, however, mitigates the presence of
carbon dioxide in the atmosphere, because this carbon dioxide is used by the crops in
photosynthesis, converting the carbon released back to biomass, in a complete carbon cycle.
The emissions of CO2 from fossil energy use, and of other greenhouse gases (notably N2O
in fertiliser manufacture and use, and CH4 from agricultural and processing operations),
should remain as low as possible. These total CO2 equivalent emissions, documented in
detail in the studies of Elsayed et al [24] and Sheehan et al [19] are:
e
44
C A  C B  CC  C D    j GWPj  X ij 
Ceq.emm. =
12
 i a

A related approach analyses avoided emissions, where the use of biomass used as fuel
replaces a quantity of fossil fuel that may have been used, or improved efficiency in energy
utilisation results in a reduction in fossil fuel use. The CO2 that may have resulted from its
combustion is classified as “avoided emissions”, and these figures would vary depending on
the energy savings calculated, as well as the measure of relativity on which they are based
(e.g., per annum, per kWh electricity produced, per hectare of land, per kilometre travelled,
etc) [25].
44
 C eq.emm.
Avoided CO2 emissions (kg CO2eq.) = C E  C cop
12


Life Cycle Assessment:
Life Cycle Assessment (LCA) is a tool for the systematic evaluation of the environmental
aspects of a product or service system through all stages of its life cycle. It provides a
holistic, i.e. cradle-to-grave, approach to evaluate environmental performance by considering
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
the potential impacts from all stages of manufacture, product use (including maintenance
and recycling), and end-of-life management. LCA provides an adequate instrument for
environmental decision support. Life cycle assessment has proven to be a valuable tool to
document the environmental considerations that need to be part of decision-making towards
sustainability. A reliable LCA performance is crucial to achieve a life-cycle economy. The
International Organization for Standardization (ISO), a world-wide federation of national
standards bodies, has standardised this framework within the ISO 14040 series on LCA [26].
There are 4 basic elements involved in conducting an LCA: 1) Definition of the goal and
scope of the study; 2) Identification and quantification of environmental loads involved; e.g.,
the energy and raw materials consumed, the air emissions, water effluents, and wastes
generated (inventory); 3) Evaluation of the potential environmental impacts of these loads
(impact assessment); and 4) Assessment of available options for reducing these
environmental impacts (interpretation).
Whilst LCA is not a single uniform approach at this time, life cycle impact assessment
(LCIA) methodology seems to be converging on similar categories [27]. The ten most
common are listed below with brief descriptions. In addition, odour, noise and radiation
effects are sometimes included, but their occurrence is not as frequent. Typical LCIA
practice employs mid-point modelling. Midpoint refers to the placement along the
stressor-impact (cause-effect) chain where the impacts are modelled. For example, the
inventory output data for different greenhouse gases is modelled to indicate potential
global warming (expressed in CO2 equivalents, then added up), not the damage caused by
climate change. In general this definition works, but it is not applicable to all impact
categories. Especially, the categories of human health and ecological health are not
considered to have a common midpoint in the cause-effect chain. This has led to the
application of various modelling approaches to these categories. Although modelling to
the endpoint results in a more environmentally-relevant and meaningful result, this level
of detail would require impossibly large amounts of time, data, resources and knowledge
of how to interpret the results. Analysis at a midpoint is an effective approach to LCIA in
that it reduces the complexity of modelling by minimizing the amount of forecasting and
effect modelling. It also results in simplifying communication of the results with fewer
categories to report.
Acidification Potential: Acidification results when sulphur dioxide and nitrogen oxides
reach the atmosphere and react with water vapour to form acids. These acids fall to earth
and can damage plants, animals, and structures. Acid deposition can occur through wet
(e.g., rain, snow, sleet), dry, or cloud water deposition (e.g., fog). Acidification compares
the capacity of substances to release hydrogen and is expressed in SO2 equivalents.
Ecological Toxicity Potential: Ecotoxicity characterization provides a relative prediction
of the potential of chemicals to cause harm to plant and animal life. Whilst determining
an ecotoxicity potential for a single chemical in a known environment is a difficult task,
expanding the list of chemicals and environments to which the modelling is applicable
makes this task even more difficult, especially since impacts of the stressors on plant and
animal species can have multiple components. A reference chemical is often selected for
the comparison, e.g., 2,4-Dichlorophenoxyacetic acid (2,4-D), and thus the units of the
ecotoxicity potentials are expressed in kg 2,4-D equivalents/kg emissions.
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
Eutrophication Potential: Eutrophication occurs when fertilizers move from land to surface
waters and cause an increase in the aquatic plant growth. This is followed by a chain of
other events including fish death, decreased biodiversity, and foul odour and taste. The
limiting nutrient is often phosphorus for freshwater systems and nitrogen for estuaries and
coastal waters, and thus the location of the release often makes a significant impact on the
relative potential for damage.
Global Warming Potential: Global warming refers to the potential change in climate that
may occur with increasing concentrations of “greenhouse gases” which trap heat that would
have otherwise passed out of the earth’s atmosphere. Resultant effects may include increased
droughts, floods, loss of polar ice caps, sea-level rise, soil moisture loss, forest loss, change in
wind and ocean patterns, and changes in agricultural production. Greenhouse gases include
carbon dioxide (CO2), methane (CH4), and nitrous oxide (NxO), as well as some compounds
that are not naturally occurring (hydrofluorocarbons (HFCs), perfluorocarbons (PFCs),
sulphur hexafluoride (SF6), etc.). The impact of a greenhouse gas is compared to the
warming potential of carbon dioxide, so global warming potential is expressed in units of
CO2 equivalents.
Human Toxicity Potential: Human toxicity characterization provides relative comparisons of
a large number of chemicals which may have the potential to contribute to cancer or other
negative human health effects. The focus of this category is not on the localised use of
chemicals within a work environment, (e.g., industrial hygiene), but the long-term exposures
to chemicals in the regional and global environment.
Ozone Depletion Potential: Ozone depletion is the reduction of the protective ozone layer
within the stratosphere caused by the emissions of ozone-depleting substances (such as
freon, chlorofluorocarbons, carbon tetrachloride, methyl chloroform, etc.). Models often
adopt the ozone depletion potentials published in the Handbook for the International
Treaties for the Protection of the Ozone Layer where chemical scores are based on CFC-11
as the reference compound.
Photochemical Ozone Creation Potential: Also known as ground-level smog, ozone is
formed within the troposphere from a variety of chemicals including nitrogen oxides, carbon
monoxide, methane, and other volatile organic compounds in the presence of high
temperatures and sunlight. High concentrations of ozone lead to negative impacts on human
health and the environment. POCP is often measured relative to ethylene and is expressed
as C2H4 equivalents.
Natural Resource Depletion: There are several ways currently being used to analyze resource
use, but no method is currently recognised as the standard methodology. Resources are any
naturally occurring material, such as ores and fossil energy sources. This category may also
include land use and water use.
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
Figure 1. Material flow and environmental interventions across the life cycle stages in
a biofuel system
Ea
Ca
Xe
Xa
Ce= carbon released
from combustion
Stage A:
Production of Inputs
Stage B:
Agriculture
&
harvesting
Cb
Eb
Enet
Eint
Xb
Es
Stage E:
Combustion at
consumer
Cs = carbon fixed during
biomass growth
Solar energy
input
Xd
Xc
Stage C:
Transport
Cc
Stage D: Ecop
Conversion
Process
Ec
Ea - Ed, E’e = fossil energy input
Ca - Cd, C’e = carbon input from fossil energy
Xa – Xe, X’e = emissions to the environment
X’e
Cd
Ed
E’e
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
Figure 2. Agricultural land efficiency of bio-ethanol in replacing fossil energy for
transportation
0
50
100
150
ligno-cellulose
200
250
300
IFEU (min)
IFEU (max)
Replaced fossil energy (GJ/ha.a)
bagasse - India
wheat straw - Great Britain
corn stover - USA
molasses - South Africa
molasses - India
corn - USA
sugarbeet - Great Britain
sugarcane - Brazil
0
50
100
150
200
Replaced fossil energy (GJ/ha.a)
250
300
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
0
5000
10000
15000
ligno-cellulose
20000
25000
30000
IFEU (min)
IFEU (max)
Avoided CO2eq. (kg/ha.a)
bagasse - India
wheat straw - Great Britain
corn stover - USA
molasses - South Africa
molasses - India
corn - USA
sugarbeet - Great Britain
sugarcane - Brazil
0
5000
10000
15000
20000
Avoided CO2 (kg/ha.a)
Figure 3. Avoided GHG emissions for different bio-ethanol systems
25000
30000
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
Table 1. Studies of biomass-to-fuel ethanol categorised by feedstock, location and
scope of the evaluation (energy/GHG or multiple criteria/LCA)
Corn
North
America
Europe
Wheat
North
America
Europe
Potatoes
Cassava
Lignocellulose
Australia
Europe
Farmed Feedstock
Energy/GHG
Multiple
criteria/LCA
Pimentel 2003 [3]
IEA 2003 [28]
Graboski 2002 [29]
USDA 2002 [13]
Berthiaume 2001 [7]
Pimentel 2001 [9]
GM 2001 [30]
Schneider 2001 [31]
Levelton 2000 [32]
Wang 1999 [33]
JRC 2003 [34]
Jungmeier 2003 [35]
Schmitz 2003 [36]
TU München 2003 [37]
IEA 2003 [28]
(S&T)2 2003 [38]
Elsayed 2003[24]
EUCAR 2003 [39]
JRC 2003 [34]
Jungmeier 2003 [35]
LowCVP 2004 [40]
Schmitz 2003 [36]
TU München 2003 [37]
Thrän 2004 [41]
ADEME 2002 [42]
CONCAWE 2002 [43]
Rosenberger 2001 [11]
Richards 2000 [44]
Hanegraaf 1998 [12]
Gover 1996 [45]
CSIRO 2001 [47]
JRC 2003 [34]
Schmitz 2003 [36]
China
Australia
Hu 2004 [18]
North
America
Europe
IEA 2003 [28]
Waste Feedstock
Energy/GHG
Multiple
criteria/LCA
Levelton 2000 [32]
Sheehan 2004 [19]
(corn stover )
(corn stover)
Elsayed 2003 [24]
(wheat straw)
Levelton 2000 [32]
(wheat straw)
Kaltschmitt 1997
[10]
Steltzer 1999 [16]
IFEU 2002 [46]
Kaltschmitt
1997
[10]
Steltzer 1999 [16]
IFEU 2002 [46]
Hu 2004 [18]
CSIRO 2001 [47]
(wood)
EUCAR 2003 [39]
(wood)
IEA 2003 [28]
(unknown)
Jungmeier 2003 [35]
LBST 2003 [48]
CONCAWE 2002 [43]
(wood & grass)
EST 2002 [49] (wood)
GM 2002 [50]
(crop residue)
EUCAR 2003 [39]
(wood & straw)
LBST 2002 [53]
(wood)
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
GM 2002 [50] (various)
JRC 2002 [51]
(wood & grass)
Fromentin 2000 [52]
Levelton 2000 [32]
(switchgrass & hay)
Philippines
Sugarcane
North
America
South
America
India
Tan & Culuba 2002
[20] (agricultural)
Bastianoni and
Marchettini 1996 [54]
Moreira 2002 [55]
Macedo 1998 [25]
Prakash 1998 [22]
(molasses)
Enerstrat 2003 [56]
(molasses)
CSIRO 2001 [47]
(molasses)
Theka 2003 [57]
(molasses)
Australia
Sugar
beet
South
Africa
Europe
Australia
Elsayed 2003 [24]
EUCAR 2003 [39]
IEA 2003 [28]
JRC 2003 [34]
Jungmeier 2003 [35]
Schmitz 2003 [36]
Thrän 2004 [41]
TU München 2003 [37]
ADEME 2002 [42]
CONCAWE 2002 [43]
GM 2002 [58]
LBST 2002 [53]
Fromentin 2000 [52]
FfE 1999 [59]
Hanegraaf 1998 [12]
CSIRO 2001 [47]
Steltzer1999 [16]
Kaltschmitt 1997
[10]
IFEU 2002 [46]
Kadam 2002 [14]
(bagasse)
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
Table 2. Bio-energy yield to fossil energy input ratios for bio-ethanol systems
Feedstock and country
Sugarcane, Brazil
Sugarbeet, Great Britain
Corn, USA
Molasses, India
Molasses, South Africa
Corn stover, USA
Wheat straw, Great Britain
Bagasse, India
Energy yield ratio
7.9
2.0
1.3
48
1.1
5.2
5.2
32
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
Table 3. Common life cycle impact categories and inventory releases for bio-ethanol compared to
conventional fuel from a review of recent literature (1996-2004)
Kaltschmitt
1997 [10]
Sugar beet
Wheat
Potato
Germany
Agricultural Feedstocks
Puppan
Reinhardt
2001[15]
2002 [17]
Sugar beet
Sugar beet
Winter wheat
Wheat
Potato
Potato
Germany
Europe
Waste Feedstocks
Hu
2004 [18]
Cassava
Kadam
2002 [14]
Waste
Bagasse
Sheehan
2004 [19]
Corn Stover
Tan & Culuba
2002 [20]
Agricultural
Cellulosic
Waste
China
India
USA
Philippines
NA
NA
NA
NA
NA
NA
NA
NA
NA
Resource
Depletion
Global
Warming
CO2
NA
NA
NA
Acidification
NA
SOx
NA
NOx
NA
Eutrophication
NA
Human
Toxicity
NA
CO
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
Accepted for Publication in the Journal of Cleaner Production (1 March 2006)
PM
Ecological
Toxicity
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
Photochemical
Smog
NA
NA
HC
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
Solid Waste
NA
Land Use
Water Use
Ozone
Depletion
Odour
NA
NA – Not Assessed
– No significant change
NA
↑
↓
– Increased impact for bio-ethanol
– Decreased impact for bio-ethanol
NA
NA
NA
NA
NA
NA
NA
NA